Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  a : A
  l : list A
  b : B
  l' : list B
  IHl : ∀ l' : list B,
          All2 R (map f l) (map g l')
          → All2 (λ (x : A) (y : B), R (f x) (g y)) l l'
  H1' : list C
  H1 : list D
  H : All2 R (pr1 {| pr1 := H1'; pr2 := H1 |})
        (pr2 {| pr1 := H1'; pr2 := H1 |})
  ============================
   ({| pr1 := f a :: map f l; pr2 := g b :: map g l' |} =
    {| pr1 := H1'; pr2 := H1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2 (λ (x : A) (y : B), R (f x) (g y)) (a :: l) (b :: l'))


DEBUG:Going to execute:
cbv beta iota delta [TODO projection TODO projection] in id, id'
DEBUG LTAC Evaluated term: H1'
DEBUG LTAC Evaluated term: (list C)
Level 4: type of id
 has value 
(list C)